In general, an automotive vehicle engine is constructed and operated in such a manner that the maximum opening amount of the valve corresponds the maximum lift amount of the cam profile of the valve opening and closing cam and is maintained constant. Under such condition, it is difficult at the present stage to ensure optimum intake and exhaust efficiencies in the respective running modes determined depending upon revolution speed of the engine and load data including the opening degrees of the throttle valves.
More specifically, the cam profile is determined in view of the fact that the fuel/air mixture intake and exhaust efficiencies should be boosted within the range of high speed condition of the engine in order to produce an engine required to have a high output, while the fuel consumption ratio tends to be sacrificed within the range of the low speed condition of the engine.
On the other hand, the cam profile is designed to have a high fuel consumption ratio which, however, sometimes does not ensure sufficient outputs of the engine during the high speed and high load conditions.
For this reason, there have been proposed a wide variety of automotive vehicle engines each having a cylinder suspending mechanism which suspends the working of partial cylinders selected from the whole cylinders of the engine so that the fuel consumption ratios of the engine under the low revolution speed and low load conditions can be reduced and the outputs of the engine can be increased under the high revolution speed and high load conditions. This kind of engine comprises a plurality of valve opening and closing cams each having high and low speed cam profiles, a plurality of rocker arms each having a cam follower held in opposing and engaging relationship to each of the cams, and a plurality of rocker shafts fixedly supporting the rocker arms, respectively, and each having a sub-rocker arm held in engaging relationship with the valve stem of the valve. The rocker arms and rocker shafts are so constructed as to be selectively engaged with and disengaged from each other by means of plungers each slidably received in the rocker arm and the rocker shaft to be hydraulically controlled.
In the foregoing cylinder suspending mechanism, the engaging rocker arms are rocked to operate the sub-rocker arms, respectively, by way of the respective rocker shafts for opening and closing the valves. One example of such an apparatus is disclosed, for example, in the specification and drawings of Japanese Patent Application No. 2-182131 filed by the present applicant.
The known apparatus is so constructed that the opening and closing amounts of the valve can be changed by the valve opening and closing cam during low and high speed operations while the rocker :arm can be engaged with and disengaged from the rocker shaft by the plunger to enable the cylinder to be held under worked and unworked conditions.
For example, the unworked cylinders such as Nos. 1 and 4 cylinders are determined for a four-cylinder engine in advance of the unworked cylinder operation of engine. The unworked cylinder operation is determined on the basis of a running mode map selected depending upon the revolution speed and load data of the engine.
The above engine having such a cylinder suspending system encounters such a difficulty that when the all cylinder working mode having the valves opened and closed in all cylinders such as for example four cylinders held under worked conditions changed to the partial cylinder unworking mode having the valves opened and closed in the only two cylinders held under unworked conditions, the vacuumization tendency in the intake system under the all cylinder working mode is larger than that under the partial cylinder working mode for the same opening degree of the throttle valve due to the increased amount of air to be introduced into the engine. This results in the fact that when the all cylinder working mode is switched to the partial cylinder unworking mode, sufficient time is required until the boost pressure reaches a certain vacuum pressure commensurate with that of the partial cylinder working mode, thereby causing a time lag and thus failing to supply an adequate amount of air to the engine.
Consequently, the output of the engine is apt to be decreased as a result of the insufficient amount of air to be supplied to the engine until the boost pressure is enhanced to the predetermined level.
On the other hand, it has been found, from the point of view for the variation characteristics of the boost pressure and the output torque of the engine at the transition time period of the running condition from the all cylinder working mode (four-cylinder working condition) to the partial cylinder unworking mode (two-cylinder working condition) in the automotive vehicle engine having such an cylinder suspending system, that the output torque of the engine is remarkably decreased (as shown by a single dot and dash line in FIG. 18) in addition to the time lag taken for rising to a predetermined boost pressure when the all cylinder working mode is changed to the partial cylinder working mode under the influence of the hydraulically driven valve mechanism.
Therefore, there has been an undesirable possibility that the torque shock takes place as a result of the abrupt decrease of the output torque of the engine at the transition time period of the running condition from the all cylinder working mode to the partial cylinder unworking mode.
On the other hand, when the partial cylinder unworking mode having the valves opened and closed in the only two cylinders held under unworked conditions is changed to the all cylinder working mode having the all valves opened and closed in the all cylinders held under worked conditions, the vacuumization tendency in the intake system under the partial cylinder unworking mode is smaller than that under the all cylinder working mode for the same opening degree of the throttle valve due to the increased amount of air to be introduced into the engine. In reality, the air volume capacity in the intake system causes a time lag until the inner pressure of the intake manifold is lowered to a predetermined vacuum level at the transition time period of the running mode.
Therefore, adequate amount of air necessary for the combustion stroke of the cylinder could be introduced into the cylinder if the partial cylinder unworking mode is switched to the all cylinder working mode to cause the inner pressure of the intake manifold to be abruptly varied, however, a large amount of air is to instantly be introduced into the engine to the degree of the time lag caused in response to the air volume capacity in the intake system by the reason described above. For this reason, the output of the engine is abnormally increased, thereby resulting in the torque shock of the engine at the transition time period of the running mode from the partial cylinder unworking condition to the all cylinder working condition.
It is therefore an object of the present invention to provide an automotive vehicle engine which is so constructed as to prevent the torque shock of the engine caused by the abnormal variation of the inner pressure of the intake manifold at the transition time period of the running mode from the partial cylinder unworking condition to the all cylinder working condition.
It is another object of the present invention to provide an automotive vehicle engine which is so constructed as to prevent the torque shock of the engine caused by the abnormal variation of the inner pressure of the intake manifold at the transition time period of the running from the all cylinder working condition to the partial cylinder unworking condition.